In many oilfield applications, reservoir fluid samples are collected and thermodynamic studies and/or other studies are performed to obtain desired information on a subterranean reservoir. The thermodynamic studies involve measuring thermodynamic properties of reservoir fluids for phase behavior analysis and/or sample validation.
Phase behavior of reservoir fluids can be characterized using a plurality of apparatus types. Generally these devices are in the form of pressure retaining vessels capable of withstanding high temperatures and pressures. The pressure retaining vessels or cells use either mercury or pistons (in the case of mercury-free cells) to impart pressure onto the fluid sample via some type of displacement pump or mechanical drive. In mercury cells the immiscibility of the mercury with the sample is exploited to simplify the design, and no piston is required to impart pressure. Mercury has several obvious drawbacks and over the last several years the industry trend generally has been to move away from such designs. In the mercury-free cells, pressure is imparted on the sample fluid via a floating piston. The piston in turn is driven/moved either mechanically or hydraulically.
The pressure retaining cells are usually a visual type or blind type and are configured with sensors for measuring pressure and temperature. The cells may also work in cooperation with measuring instruments and/or sensors for measuring total sample volume, phase volumes, saturation pressures, and other parameters, either with the sensor or visually by an operator. In some cases ancillary external equipment can be configured in conjunction with the cells to make additional measurements such as density and viscosity, in which case a larger sample volume of fluid is required to make the additional measurements. Often, the external equipment can be operated in stand alone mode to make these measurements independent of the cell. The cells may have some mechanism for allowing a sample to be extracted during the experiment under equilibrium conditions via, for example, a sampling valve.
In addition to pressure management and experimental measurement sensors, the devices may have some type of thermal management system for temperature control, e.g. ovens or heating mantles/jackets. The equilibrium cell may also work in cooperation with a mechanism for agitating the sample. This is done to speed up the equilibrium process and hence increase experimental efficiency. The types of agitation mechanisms include magnetically coupled mechanical impeller type mixers, simple rocking mechanisms (with or without mixing rings), circulation pumps, and ultrasonic transducers.
The equilibrium cells are often designed specifically for the type of fluid under study. For example it is common to use a conical piston for the study of gas condensates and a flat piston for oils. The conical pistons are employed because the amount of liquid dropout from gas condensates is very small and by using conical pistons the capability of the apparatus to measure these very small volumes is enhanced.
Another trend to enhance the study of gas condensates is to use equilibrium cells with larger volumes than those used for oil studies. The rationale is that the larger the sample volume the greater the liquid dropout volume, which increases the likelihood of being within the measuring resolution of the instruments. One of the major drawbacks of these larger cells is the requirement of a larger sample volume.
Density and viscosity measurements may be performed by other pieces of equipment external to the main cell, e.g the PVT (pressure-volume-temperature) cell, or by incorporating a densitometer or viscometer into the apparatus. One common form of viscometer incorporated into the cell uses a capillary technique, and the most common form of densitometer is based on a vibrating tube technique. An example of such a densitometer is that made by Anton Paar GmbH of Graz, Austria. These measurement devices require that the sample is flowed/pushed through the viscometer or densitometer and, as such, require substantial sample volume to flow through the sensor for measurement and to flush/clean the sensors. These flow-through type sensors have many drawbacks, including a relatively large equipment footprint and sample volume requirement.
To determine phase volumes, most apparatus types measure the gas-liquid interface. The gas-liquid interface is formed as a result of being in a region of the phase envelope below the saturation point and having the gas and liquid layers stratified within the cell body. It is important that the gas phase and liquid phase be in equilibrium. Stratification will occur naturally, but this can take several hours, days, or weeks depending on the fluid system. In order to increase experimental efficiency, agitation is used to significantly reduce the time needed to reach equilibrium to the order of seconds or minutes. This requires the gas-liquid contact area to be maximized, sufficient gas-liquid retention time, and movement of both phases for the mass diffusion between the phases to be maximized at a given temperature and pressure.
When equilibrium is achieved the mass transfer of the individual components into each of the respective phases becomes zero. This is due to the conditions of thermodynamic equilibrium where the temperature and pressures of each phase are identical and the chemical potentials or fugacities of each component within each phase also become identical. An agitation or mixing technique is the standard technique used for decreasing approach times to equilibrium, the most effective being recirculation of one phase thorough the other. Agitation systems are varied, and include magnetically driven mixing rings/pistons/devices, simple cell rocking, a combination of mixing rings/pistons/devices and rocking, magnetically coupled impeller mixers, magnetic stirrers, static mixers, orifice mixers, circulation pumps, and ultrasonic stirrers (clamp-on externally mounted or transducer direct contact types).
In any case, existing devices lack sufficient sensor capabilities or combinations of sensor capabilities to enable sufficient phase behavior and sample validation studies of reservoir fluids.